Am 28.02.2012 18:56, schrieb Eric Firing: > On 02/28/2012 06:28 AM, Andreas H. wrote: >>>> On Tuesday, February 28, 2012, Andreas H. wrote: >>>> >>>>> Good morning, >>>>> >>>>> I'm creating the attached plot using pcolormesh(). What I would like to >>>>> do now is draw contour lines at +/- 2.5%, which follow the grid edges. >>>>> >>>>> The problem is that when I use contour(), the lines drawn do not follow >>>>> the grid edges but seem to be interpolated somehow. >>>>> >>>>> Do you have an idea how to draw the contour lines following the grid >>>>> edges? >>>>> >>>>> Your insight is very much appreciated :) >>>>> >>>>> Cheers, >>>>> Andreas. >>>>> >>>> >>>> This is because of a subtle difference in how pcolor-like functions and >>>> contour-like functions work. I always forget which is which, but one >>>> assumes that the z value lies on the vertices of the grid while the >>>> other >>>> assumes that it lies in the middle of each grid point. This is why you >>>> see >>>> them slightly offset from each other. >>> >>> Thanks, Ben! >>> >>> To `pcolormesh`, I pass the *edges* of the grid: >>> >>> xbin = linspace(0, 12, nxbin + 1) >>> ybin = np.linspace(-90, 90, nybin + 1) >>> >>> pl = spl.pcolormesh(xbin, ybin, pdata.T, cmap=cmap, edgecolors='None', >>> vmin=-5, vmax=20) >>> >>> `contour`, however, wants the coordinates themselves. So I do >>> >>> spl.contour((xbin[:-1]+xbin[1:])/2., (ybin[:-1]+ybin[1:])/2, pdata.T, >>> [-2.5, 2.5]) >>> >>> Still, the outcome is, well, unexpected to me. Actually, no matter if >>> contour wants centres or edges, the actual behaviour seems strange. There >>> is some interpolation going on, apparently. The input `pdata` has shape >>> (12, 72) (or 72,12), and I definitely wouldn't expect this sub-grid >>> movement in the x-direction. >>> >>> Any ideas? >> >> Okay, after some diving into matplotlib sources, I guess the interpolation >> comes within the function `QuadContourSet._get_allsegs_and_allkinds`. So >> there seems to be no way to accomplish what I actually want with the >> current matplotlib API. Correct? >> >> If I wanted to do something about this, I would need to >> >> * implement a class `GriddedContourSet`, derived from `ContourSet`, where >> I implement the `_get_allsegs_and_allkinds` method appropriately. >> * add an additional keyword argument to `contour()` to make this gridded >> contourset an option when calling `contour()`. >> >> Is this all correct? If yes, I might start working on this if I get the >> time ... > > It is not at all clear to me what you want to do, as compared to what > contour does. Can you illustrate with an extremely simple example? > Maybe even a scanned sketch, if necessary? Do you want the contour lines > to be stepped, like the rectilinear boundaries of the pcolormesh > cells--that is, composed entirely of horizontal and vertical line segments?
Yes, Eric, that's exactly what I want. Since my case was simple enough, I did it completely manually, with loads of calls to `plot` (I'm sure there would've been a simpler solution ... -- which one?). I attached the plot so you get an idea of what I want to do. Thanks for your help! Andreas.
example.pdf
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